Re: [patch v2 3/6] memcg: rework mem_cgroup_iter to use cgroup iterators

[Michal Hocko <mhocko@suse.cz>]

On Wed 12-12-12 20:24:41, Michal Hocko wrote:
> On Wed 12-12-12 10:06:52, Michal Hocko wrote:
> > On Tue 11-12-12 14:36:10, Ying Han wrote:
> [...]
> > > One exception is mem_cgroup_iter_break(), where the loop terminates
> > > with *leaked* refcnt and that is what the iter_break() needs to clean
> > > up. We can not rely on the next caller of the loop since it might
> > > never happen.
> > 
> > Yes, this is true and I already have a half baked patch for that. I
> > haven't posted it yet but it basically checks all node-zone-prio
> > last_visited and removes itself from them on the way out in pre_destroy
> > callback (I just need to cleanup "find a new last_visited" part and will
> > post it).
> 
> And a half baked patch - just compile tested

please ignore this patch. It is totally bogus.

> ---
> From 1c976c079c383175c679e00115aee0ab8e215bf2 Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@suse.cz>
> Date: Tue, 11 Dec 2012 21:02:39 +0100
> Subject: [PATCH] NOT READY YET - just compile tested
> 
> memcg: remove memcg from the reclaim iterators
> 
> Now that per-node-zone-priority iterator caches memory cgroups rather
> than their css ids we have to be careful and remove them from the
> iterator when they are on the way out otherwise they might hang for
> unbounded amount of time (until the global reclaim triggers the zone
> under priority to find out the group is dead and let it to find the
> final rest).
> 
> This is solved by hooking into mem_cgroup_pre_destroy and checking all
> per-node-zone-priority iterators. If the current memcg is found in
> iter->last_visited then it is replaced by its left sibling or its parent
> otherwise. This guarantees that no group gets more reclaiming than
> necessary and the next iteration will continue seemingly.
> 
> Spotted-by: Ying Han <yinghan@google.com>
> Not-signed-off-by-yet: Michal Hocko <mhocko@suse.cz>
> ---
>  mm/memcontrol.c |   38 ++++++++++++++++++++++++++++++++++++++
>  1 file changed, 38 insertions(+)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 7134148..286db74 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6213,12 +6213,50 @@ free_out:
>  	return ERR_PTR(error);
>  }
>  
> +static void mem_cgroup_remove_cached(struct mem_cgroup *memcg)
> +{
> +	int node, zone;
> +
> +	for_each_node(node) {
> +		struct mem_cgroup_per_node *pn = memcg->info.nodeinfo[node];
> +		int prio;
> +
> +		for (zone = 0; zone < MAX_NR_ZONES; zone++) {
> +			struct mem_cgroup_per_zone *mz;
> +
> +			mz = &pn->zoneinfo[zone];
> +			for (prio = 0; prio < DEF_PRIORITY + 1; prio++) {
> +				struct mem_cgroup_reclaim_iter *iter;
> +
> +				iter = &mz->reclaim_iter[prio];
> +				rcu_read_lock();
> +				spin_lock(&iter->iter_lock);
> +				if (iter->last_visited == memcg) {
> +					struct cgroup *cgroup, *prev;
> +
> +					cgroup = memcg->css.cgroup;
> +					prev = list_entry_rcu(cgroup->sibling.prev, struct cgroup, sibling);
> +					if (&prev->sibling == &prev->parent->children)
> +						prev = prev->parent;
> +					iter->last_visited = mem_cgroup_from_cont(prev);
> +
> +					/* TODO can we do this? */
> +					css_put(&memcg->css);
> +				}
> +				spin_unlock(&iter->iter_lock);
> +				rcu_read_unlock();
> +			}
> +		}
> +	}
> +}
> +
>  static void mem_cgroup_pre_destroy(struct cgroup *cont)
>  {
>  	struct mem_cgroup *memcg = mem_cgroup_from_cont(cont);
>  
>  	mem_cgroup_reparent_charges(memcg);
>  	mem_cgroup_destroy_all_caches(memcg);
> +	mem_cgroup_remove_cached(memcg);
>  }
>  
>  static void mem_cgroup_destroy(struct cgroup *cont)
> -- 
> 1.7.10.4
> 
> -- 
> Michal Hocko
> SUSE Labs
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-- 
Michal Hocko
SUSE Labs

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